1. Field of the Invention
The present invention relates to a lightning-protective fastener to prevent explosion for use in a portion having a possibility of being filled with flammable fuel vapor of a fuel tank or the like placed in an airframe of an aircraft, in particular, a wing or a fuselage.
2. Description of the Related Art
A wing configuring an airframe of an aircraft generally has a hollow structure, and a wing-surface panel forming a wing surface is fixed to a structural member inside of a wing with fastener members (stoppers).
Here, a fastener member has a pin-shaped fastener body inserted from outside of the wing into through holes formed on both of the wing-surface panel and the structural member inside of the wing and, with its tip fixed from inside of the wing with a fixing metal part, thereby fastening the wing-surface panel and the structural member together.
In addition, also in the inside of the wing and a fuselage, structural members other than the wing-surface panel and members for fixing equipment are fastened and fixed with fastener members.
Here, in a fastener member, the pin-shaped fastener body is inserted so as to pass through both through holes formed in both members to be fixed together and has its tip fixed to a fastening member (a collar), thereby fastening both of the members together.
Note that the number of wing-surface panels or members to be fixed is not restricted to two.
Meanwhile, in an aircraft, it is required to fully assure lightning-protective measures for preventing explosion. If lightning hits the aircraft and a large current flows through the wing-surface panel or a structural member of a main wing or the like, part or an entirety in some cases of the current flows through fastening parts with the fastener body and the fastening member described above. If the current value exceeds a limit value of a passage allowable current at each fastening part, an electric discharge called an electric arc (or a thermal spark) occurs (hereinafter referred in the specification as an arc). This is a phenomenon in which a local part at a fastening interface between members mainly made of a conductive material configuring the fastening part has an occurrence of an abrupt increase in temperature due to a current passing through the fastening part, and is molten to cause an electric discharge into the air. In many cases, a molten substance called hot particles is scattered from a molten portion. In general, since the inner space of the wing is used also as a fuel tank, an explosion prevention measure is required, in which ignition is prevented by suppressing the occurrence of an arc or sealing against an arc at the time of a hit by lightning, thereby preventing a discharge of the generated arc and hot particles scattered therefrom from being in contact with flammable fuel vapor. Here, examples of the portion having a possibility of being filled with flammable fuel vapor include the inside of a fuel tank, the inside of a surge tank (a tank where a vent scoop, a burst disk, and others are placed) generally placed on a wing end side of the fuel tank, and the inside of fuel-system equipment, in a portion inside the wing and a fuselage part.
Here, measures of suppressing the occurrence of an arc and sealing against the generated arc need to be considered for coupling interfaces between all portions of the fastener body and the fastening member and the wing-surface panel and the structural member in contact with these portions.
Among these, in a coupling interface between the fastening member and a structural member, a washer and a spacer made of an insulating material are conventionally interposed, thereby insulating the coupling interface between a bearing surface of the fastening member and a surface of the structural member. With this, the current at this portion is interrupted, and the effect of suppressing the occurrence of an arc is reinforced (for example, refer to Japanese Patent Application Laid-Open No. 2009-227166).
However, in the technology described in Japanese Patent Application Laid-Open No. 2009-227166, since the washer and and the spacer are interposed between the fastening member and the structural member, the technology has a problem such that a fastening force by the fastener member and the fastening member cannot be sufficiently achieved, providing poor feedback when the fastening member is mounted on the fastener member.
In other words, the problem described above is as follows. When the washer and the spacer are formed of an insulating material, a resin-based material is generally adopted. If the washer and the spacer are formed of a resin-based material, when the fastener member and the fastening member are fastened together, the washer and the spacer are deformed in their thickness direction. If the fastening force is increased, the washer and the spacer are crushed, and the fastening force cannot be sufficiently achieved.
Also, with the washer and the spacer being deformed at the time of fastening, tactile feedback when mounting the fastening member on the fastener member is impaired. This makes it difficult for a worker to ascertain to which degree the fastening member is screwed into, thereby impairing work stability.
Further, if the worker forgets to mount these washer and spacer, it is disadvantageously impossible to achieve an effect of reinforcing suppression of the occurrence of an arc by interrupting current with these washer and spacer.
Also, when a hole wall surface of a structural member is coated with an insulating coating and an insulating surface treatment is further performed on the fastener member, the fastener member or the fastening member and the structural member are not electrically connected. In such case, an electric field concentrates on an outer circumferential edge part of the fastening member, and insulation breakdown may occur between the outer circumferential edge part of the fastening member and the structural member to cause a spark.